Shouldn't it be possible to judge most morality on the basis of these common features, making an argument like "wanton murder is bad, because it goes against the empathy your brain evolved to feel, and hurts the survival chance you are born valuing"?

Yes, it should. However, in the hypothetical case involved, the reason is not true; the hypothetical brain does not have the quality "Has empathy and values survival and survival is impaired by murder".

We are left with the simple truth that evolution (including memetic evolution) selects for things which produce offspring that imitate them, and "Has a moral system that prohibits murder" is a quality that successfully creates offspring that typically have the quality "Has a moral system that prohibits murder".

The different quality "Commits wanton murder" is less successful at creating offspring in modern society, because convicted murderers don't get to teach children that committing wanton murder is something to do.